1. Field of the Invention
The invention relates to solder compositions, in particular solder compositions useful in precision optical and opto-electronic applications.
2. Discussion of the Related Art
One of the challenges for optical and opto-electronic packaging is to connect components with precise alignment, and to maintain the stability of the alignment during device operation, i.e., through fluctuations in the ambient temperature and stress conditions at the interconnect. In addition, where the components are highly stress-sensitive, it is desirable to reduce stresses in the solder joint.
For example, precision coupling is necessary between optical fibers or waveguides and optically active devices such as lasers, light-emitting diodes, or photodetectors. Such coupling is often accomplished by active alignment, e.g., by monitoring the coupling efficiency between a fiber and a laser, adjusting the position of the fiber and laser to reach the desired efficiency, and then proceeding with a permanent assembly procedure such as soldering, brazing, or adhesive bonding. Because active alignment is inefficient and costly, passive alignment techniques, which use, for example, solder self-alignment alignment or fixtures such as v-grooves, are often more desirable. In both alignment schemes, but particularly passive alignment, the dimensional stability of the alignment is crucial. Creep deformation of solder joints will lead to a permanent shift of the optical alignment between the mating devices, causing deterioration of optical efficiencies. Thus, solder joints for such optical applications are desirably formed from solder having high creep resistance.
However, highly creep resistant solders generally exhibit relatively high stresses. Specifically, such solders tend to lock in stresses created during solder joint formation, e.g., due to thermal expansion mismatches between the solder and the components being joined, or induced by external mechanical force. Such high stresses are not compatible with some optical applications. For example, polarization maintaining fiber (PMF) is currently of interest due to its ability to enhance the performance of various optical communication systems and sensor devices. Polarization maintenance in PMF is achieved by constructing the optical fiber such that the two orthogonal components of the fundamental mode experience different propagation constants, either by inducing birefringence of the fiber material or by introducing geometrical ellipticity in the fiber core. (See, e.g., R. E. Thompson and F. I. Akers, xe2x80x9cPolarization maintaining single-mode fibersxe2x80x9d, IOOC ""81, Third International Conference on Integrated Optics and Optical Fiber Communication Technical Digest, 1981, p.60.) These construction techniques make PMF highly sensitive to stresses exerted on the fiber, in that stresses are able to affect both the birefringence profile and the geometrical ellipticity. Therefore, bonding of PMF to active devices requires solder joints that are not only creep-resistantxe2x80x94to maintain alignment, but also exhibit low stresses on the PMF. Unfortunately, as noted above, the properties of low stress and high creep resistance tend to be mutually exclusive, in that highly creep-resistant solders tend to be non-ductile and thus hold in stresses, while lower-stress solders tend to have higher ductility and thus lower creep resistance.
For these reasons, it would be desirable to have solders more suitable for optical and opto-electronic applications, i.e., solders that exhibit a combination of high creep resistance but also relatively low induced stress. Current solders do not meet these requirements. Specifically, typical microelectronic solder such as the 37Pbxe2x80x9463Sn eutectic solder has a relatively low melting temperature of about 183xc2x0 C. Under the usual temperature cycling, and in the presence of stresses in the solder joints (from thermal expansion mismatches or from mechanical force), creep deformation of such Pbxe2x80x94Sn solder is unavoidable, and the associated degradation in optical alignment is typically unacceptable.
The opposite problem is encountered with Auxe2x80x94Sn solders. Because higher melting point solders generally have higher creep resistance, solder based on the Auxe2x80x94Sn system (melting point of 280xc2x0 C. at a eutectic composition of 80 wt. % Auxe2x80x9420 wt. % Sn) is conventionally used for most opto-electronic applications, and is, in fact, one of the most creep-resistant solders known. However, as shown in the Auxe2x80x94Sn phase diagram of FIG. 1, the 80 wt. % Auxe2x80x9420 wt. % Sn composition corresponds to a eutectic between two intermetallic phasesxe2x80x94Au5Sn and AuSn, and is therefore relatively hard and brittle. This lack of ductility tends to create a significant amount of stress in the solder joints, particularly during the cooling from the relatively high soldering temperature (typically 300-310xc2x0 C.). Thus, while such a Auxe2x80x94Sn solder exhibits desirable creep resistance, its stress-related properties make it undesirable for bonding stress-sensitive components such as PMF. In addition, the high melting point increases the difficulty of the bonding and assembly steps.
Thus, there remains a need for relatively low melting temperature solders which exhibit relatively high creep resistance at operating temperaturesxe2x80x94to maintain the stability of an opto-electronic assembly, yet exhibit relatively low stresses in the solder joints.
The invention relates to use of a solder composition exhibiting a desired combination of high creep resistance at typical operating temperatures and low stress in formed solder joints. The invention uses a solder containing 82 to 85 wt. % Au, 12 to 14 wt. % Sn, and 3 to 4 wt. % Ga (optionally with up to 2 wt. % additional elements). Ga exhibits unique properties such as low melting point (29.76xc2x0 C.), low vapor pressure even at high temperatures, and high resistance to attack by acids. These characteristics allow a small amount of added Ga to induce a significant depression in the liquidus temperatures of both Au and Sn (see FIGS. 2A and 2B), and thus a depressed melting point (at least 10xc2x0 C. less, typically about 27xc2x0 C. less, than eutectic Auxe2x80x94Sn solder). The Ga also provides enhanced temperature-sensitivity of the solder""s creep resistance.
The Ga-containing solder meets the conflicting demands of high creep-resistance and stress-relaxation of solder joints in the following manner. At typical operating temperatures, generally below a temperature of 170 to 230xc2x0 C., the Auxe2x80x94Snxe2x80x94Ga solder exhibits higher creep resistance, relative to 80wt. % Auxe2x80x9420wt. % Sn. This high creep resistance thereby maintains desirable component alignment under typical operating conditions. However, at higher temperatures at which reflow and initial cooling from reflow occur, the solder exhibits lower creep resistance compared to 80 wt. % Auxe2x80x9420 wt. % Sn, this lower creep resistance allowing for relaxation of stresses during cooling from reflow. The resultant solder joint thereby encounters less stresses in use, but is able to sufficiently maintain the alignment of optical components under operating temperatures.